As computer system frequencies and power increase, the electromagnetic energy or noise, also known as electromagnetic interference (EMI), radiating from the computer system housing increases proportionally. This EMI noise typically originates from the different components within the computer system, and can interfere with the operation of other pieces of electronic equipment associated with the computer system or in close proximity thereto. As such, computer systems typically need to comply to an electromagnetic compliance (EMC) standard which defines limits to levels of stray EMI noise. To comply with EMC standards and to protect electronic equipment from operation affecting EMI noise, the computer system components that emit EMI noise are generally placed within a shielded enclosure, such as a computer system housing. The computer system housing suppresses EMI noise emanating from computer system components by containing or otherwise diminishing stray EMI noise signals.
One type of computer system housing that attenuates EMI noise employs a PCB input/output (I/O) connection system known as the peripheral component interconnect (PCI) system. The popularity of the PCI system has grown to a point where it is the preferred I/O connection system for larger server computers as well as workstation computers. The PCI system allows one or more microprocessors (i.e., PCB's) to be interconnected with attached external devices.
The PCI system includes a computer system housing defined by a plurality of walls. At least one of these walls is removable to provide access to an interior region of the housing to allow PCB's to be inserted into and removed from the housing. The PCB's are releasably connectable to a central processing unit of the computer system through a mother board, often referred to as a backplane board, as it is vertically mounted as a back wall of the computer system housing.
A wall of the computer system housing adjacent to the backplane board includes a plurality of PCI slots (i.e., between six and twelve total) which are disposed in the computer system housing to provide external access to the PCB's mounted within the housing. Peripheral component connectors on the PCB's extend through these PCI slots so that compatible external devices can be plugged into the PCB via the peripheral component connectors. This wall that includes the PCI slots is often referred to as the bulkhead wall of the computer system housing. In the PCI system, that portion of a PCB that includes the peripheral component connector also includes a bulkhead plate that covers the portion of the respective PCI slot not taken up by the peripheral component connector. In addition, in the PCI system, those PCI slots not currently accommodating a PCB are covered by individual blank bulkhead plates that are substantially identical to the PCB bulkhead plates and function to cover the unused PCI slots. The PCB and blank bulkhead plates are typically secured to the housing, and in particular, to the bulkhead wall via a separate screw fastener.
The walls of the PCI computer system housing generally provide some suppression of the EMI noise produced by the PCB's mounted within the housing. However, EMI noise from the PCB's does radiate from the PCI slots to such an extent that a typical PCI computer system housing does not meet EMC standards. To provide conduction paths to the computer system housing for EMI noise currents to meet EMC standards, bulkhead gaskets are used.
In the typical PCI system, the bulkhead gasket is placed on the interior surface of the bulkhead wall. The bulkhead gasket makes contact with the bulkhead wall and with the bulkhead plates along the two longest sides of the bulkhead plates. Typically, the bulkhead gasket is a thin (0.004") stainless steel metal stamping containing a line of fonned spring tabs as contact points between the bulkhead wall and the bulkhead plates. Because typical PCI slots are arranged close together on 0.80" centers, there is limited space between bulkhead plates. As such, only enough room exits on the bulkhead gasket for either a single row of spring tab contacts which adjacent bulkhead plates must share, or an interlaced pattern of spring tabs which adjacent bulkhead plates do not share. With either arrangement, if adjacent bulkhead plates are misaligned relative to one another, one bulkhead plate will make little or no contact with the bulkhead gasket. Moreover, since this type of bulkhead gasket only contacts the bulkhead plates along their two longest sides, large non-contact gaps are left between the two shortest sides of the bulkhead plates and the bulkhead wall resulting in EMI noise leakage. Without contact or proper contact between the bulkhead plates and the bulkhead gasket, EMI noise will not be conducted to the computer system housing, and as such, the EMI noise will not be suppressed.
As set forth previously, each of the PCB and blank bulkhead plates are typically secured to the housing, and in particular, to the bulkhead wall via a single, separate screw fastener. Each screw passes through a slot within a mounting tab that is integral with and extends at a 90.degree. angle with respect to a main portion of the bulkhead plate. This screw/mounting tab slot configuration must secure and align the bulkhead plate within the PCI computer system housing. Because of this screw/slot mounting arrangement, the close center spacing of the PCI slots, and because the material thickness of the bulkhead plates is very thin (i.e., 0.030"), the bulkhead plates tend to misalign and overlap each other. This misalignment and overlap of the bulkhead plates results in no contact and/or improper contact between the bulkhead plates and the bulkhead gasket. As such EMI noise will not be suppressed, since the EMI noise is properly conducted to the computer system housing.
In addition, the single screw fastener bulkhead plate mounting arrangement exhibits other drawbacks. In particular, the loose screw is susceptible to being lost, or worse, being dropped into the computer system housing. As such, to insert and remove PCB's from the typical PCI computer system housing, power to the computer system must be turned off or a repair technician risks an electrical short caused by a loose screw being dropped into the housing. Hence, the PCI computer system housing described above is not amenable to "hot plugging" of PCB's (i.e., the installation and removal of PCB's to and from the computer system housing with the power to the computer system turned on.
There is a need for an improved PCI computer system housing. In particular, there is a need for a PCI computer system housing that employs a bulkhead gasket assembly that suppresses EMI noise along the entire peripheral edges of the bulkhead plates. In addition, the PCI computer system housing should employ a bulkhead plate alignment and mounting system that properly aligns the bulkhead plates to insure adequate contact between the bulkhead plates and the bulkhead gasket to attenuate EMI noise. Moreover, the bulkhead plate alignment and mounting system should be free of loose fasteners to allow hot plugging of PCB's.